Since the discovery of oriented linear condensation polymers, as described by Carothers in U.S. Pat. No. 2,071 250, oriented polymeric monofilaments have been used in a wide variety of applications. Carothers himself suggested a number of uses, including artificial hair bristles, threads, filaments, yarns, strips, films, bands and the like. Many of these uses have, in fact, found their way into commercial products, such as components of textile yarns, brush bristles and fishing line. However, it has previously been impossible to prepare oriented polymeric monofilaments having a large diameter, that is, greater than about 50 mils or about 1.27 mm.
The difficulty in preparing such large diameter monofilaments lies in the cooling of such materials after melt extrusion. The nature of such polymers is such that a density differential is created within an extruded structure upon cooling from the plastic to the solid state. Quenching of the filament initially solidifies the outer surface, with a rearrangement of the structure to accommodate the shrinking of the outer surface as it solidifies, at which point the center is still in a molten state. As the center solidifies, it also shrinks, leaving voids in the structure. While this effect is not substantial in small monofilaments, with increasing diameters of monofilament, this density differential from exterior to interior creates voids which either make polymer orientation impossible or result in oriented products with tensile properties that severely restrict end uses. Moreover, large diameter polymeric monofilaments are typically irregular in cross-sectional configuration, being characterized by an ovality that is unacceptable in many applications.
Previous attempts to prepare large diameter monofilaments have included the extrusion of a hollow filament, as described in U.S. Pat. No. 3,630,824. While the possibility of preparing large diameter structures is suggested in that patent, similar difficulties are realized with increasing wall thickness of the hollow filament. Other solutions that are less than completely satisfactory include the use of a polymer having an exceptionally low rate of crystallization, so that the change in density is so gradual that the expected void formation is not realized.
The previous inability to prepare large diameter monofilaments of semi-crystalline, linear polymers has resulted in the use of other shapes, such as flat tapes, when the desirable properties of a polymer were needed combined with tensile requirements that exceeded those that could be obtained with monofilaments. However, a need remains for large diameter monofilaments, particularly in applications where available space does not permit the use of a wide tape.